This invention relates to improved units adapted to be connected into a well drilling string and constructed to pass a cable from the exterior of the string to its interior at the location of the unit.
During the drilling of a well, it is frequently desirable to utilize an instrument or other equipment carried by a lower portion of the drill string and connected to the surface of the earth by an electrical cable. For example, the drill string may contain a `steering tool` probe adapted to sense the inclination of the lower portion of the drill string and the direction of that inclination and convey the sensed information to the surface of the earth through a cable to indicate to the driller the amount and direction of inclination so that he may introduce such corrections into the direction in which the drilling operation is progressing to ultimately arrive at a desired location in the earth. In other instances, a drilling motor for driving the bit may be energized or controlled through a cable.
If the cable of such a steering tool or other instrument or piece of equipment carried within or by the lower portion of a drill string extends upwardly through the interior of the string to the surface of the earth, a problem arises in connecting additional pipe sections to the upper end of the string without encountering interference as a result of the presence of the cable in the string. Unless other provision is made for avoiding this difficulty, it may be necessary to withdraw the entire cable and instrument assembly upwardly from the string each time that a section of pipe is added, and then relower the cable and instrument into the string after the section has been added. Such a procedure is extremely time consuming and expensive of rig time and labor and adds considerably to the overall cost of drilling a well.
One expedient for attempting to overcome this difficulty is disclosed in U.S. Pat. No. 4,062,551 issued Dec. 13, 1977 to Base on "Cable Seal Unit for Earth-Boring Drill Strings", which shows a tubular part adapted to be connected into the drill string at a location spaced beneath the surface of the earth and containing an inclined passageway extending through a side wall of the tubular part to pass an electrical cable through that passageway from the exterior to the interior of the part. The cable extends downwardly along the outside of the drill string to the location of this part or sub, and at that location extends through the wall of the sub to its interior to then pass downwardly within the lower portion of the drill string to the associated instrument, probe, or the like. Because the upper portion of the cable is located at the outside of the drill string, sections can be added to or removed from the upper end of the drill string without removing the cable and instrument or encountering interference by virtue of the presence of the cable. The patent contemplates sealing the wall passage through which the cable extends, and attaching a clamp to the outside of the wall of the sub part to secure the cable in place.
A difficulty which is inherent in use of a sub of the type shown in U.S. Pat. No. 4,062,551 resides in the inconvenience involved in threading the cable through the passage in the side wall of the sub. Besides that inconvenience, both the cable and the sub may be subjected to substantial wear as the cable is threaded through the side wall passage, with a resultant reduction in the effective useful life of the parts and possible early development of interruptions in the electrical insulation on the cable.